The invention relates to a moistening arrangement of a multiple-nip calender.
Multiple-nip calenders comprise a plurality of nips formed between a polymer roll and a heated steel roll. While passing between the roll nips, the fibre web is calendered at its surface. Should it be desirable to calender both sides of the fibre web with a multiple-nip calender disposed in one frame, a “reversing nip” comprising two successive polymer rolls must be provided in the multiple-nip calender. The calendered fibre web should have as equal quality as possible both in the lateral and the longitudinal direction of the fibre web, and the fibre web consequently requires moistening both before entering the multiple-nip calender and while proceeding in the multiple-nip calender. Usually one to four fibre web damping units are disposed in a multiple-nip calender, the number of damping units depending principally on the number of nips in the calender and on the fact whether the calender has been disposed in one or two frames. With an increase of the number of roll nips in multiple-nip calenders, the fibre web will require increased damping. A reversing nip also requires increased damping of the fibre web, because both sides of the fibre web will require damping: usually damping is carried out in multiple-nip calenders equipped with a reversing nip just before the fibre web enters the multiple-nip calender, before the reversing nip. Damping that takes place before the reversing nip is usually performed by means of a damping unit disposed in “a pocket” between the output roll and the set of rolls. In multiple-nip calenders, the set of rolls is usually placed within a very narrow area, and for the fibre web to be efficiently moistened in the pocket, the number of damping units must be increased. The narrow pocket cannot accommodate very large and efficient damping units, because it already accommodates the crown variation means of the idle roll, and also auxiliary means such as steam boxes, in addition to the piping of the fibre web damping unit. Any attempts to solve the problem by enlarging the pocket with the output roll transferred further away from the line of the aligned set of rolls would risk generating vibrations caused by the increased torque of the output roll.
The purpose of the invention is to eliminate the prior art shortcomings described above. Consequently, the chief purpose of the invention is to provide a fibre web moistening arrangement in a multiple-nip calender equipped with a reversing nip, the moistening arrangement allowing for efficient location of the damping unit in the pocket of the multiple-nip calender.
The invention has the additional purpose of decreasing the number of fibre web damping units in multiple-nip calenders.